Pneumatic servo or pilot-actuated control valves have a variety of applications. Those designed for square wave regulation may be used for conserving devices (e.g. U.S. Pat. No. 5,360,000 or U.S. Pat. No. 6,484,721), nebulisers and the like or for constructing pneumatic logic circuits. Alternatively, such valves find application in alarm signal generation in which, for example, a constant pressure signal is converted to a pulsed flow for feeding an alerting device, such as a whistle, to provide a pulsed alarm. Or in resuscitation apparatus in which a constant pressure signal is converted to a pulsed flow of oxygen to the lungs. Valves designed for proportional use may be used in breathing apparatus (e.g. EP 0606098 or US 2004/244797).
These valves employ a component sealed around its periphery, and pressure (control pressure) is used on one side of a central section to effect operation. Increasing pressure results in a force being applied to the central section, which is thereby displaced in order to open or close a valve or to apply a load. Valve actuation is typically on the opposite side to which the pressure is applied.
A piston or diaphragm is conventionally used in a pneumatic valve assembly as the actuation means. A piston valve comprises a moving seal such as an o-ring operating in a bore. Friction inevitably affects such movement however, which will introduce hysteresis into the action of the valve. This renders the piston generally unsuitable for use in servo or pilot-actuated control valves, particularly if a small device size is important.
More common therefore is the diaphragm. Conventionally a rubber disc is secured around its circumference with its central portion forming a seal with a seat on an inlet jet. The valve is opened or closed by flexing the central portion of the disc.
The terms “disc” and “diaphragm” are often used interchangeably in the art. In what follows however, the term “diaphragm” will be used if referring to that part of the disc that flexes against the seat in order to operate the valve. The term “disc” will be retained if referring to the entire component, which is held in the valve assembly about its periphery and flexes at its central (diaphragm) portion.
Two common means of sealing the outside of the disc are employed in control valves of this type. First, the outer perimeter may seal by interference against the inside diameter of a bore. By interference, it is meant that the outer part of the disc is distorted inwards by its fit in the bore and a reaction force radially against the bore holds the disc in place. Secondly, the outer perimeter may be clamped between two surfaces, compressing transversely to the disc, leaving the working part of the disc (including diaphragm) in the bore. The arrangement by which the disc is held and sealed affects the way in which the diaphragm behaves, and the choice of which to adopt may be made according to the intended valve application.
In a typical valve assembly, the disc is located and sealed in a bore, with a control volume on one side and a gas inlet in fluid communication with an outlet on the opposite side. The path between gas inlet and outlet may be either open or closed, depending on the flexing of the diaphragm. In operation, gas is forced in or removed from the control volume to generate a variable control pressure therein (although there may be pressure affecting movement of the diaphragm on both sides). The valve is constructed in such a way that when the control pressure is above a threshold, the device is switched and the path between the inlet and outlet may be closed or, alternatively, opened.
In oxygen conserving devices such as that described in U.S. Pat. No. 5,360,000, the disc is used in such a way that a falling pressure on the control side of the valve can cause the valve to open. A threshold (higher) pressure on the control side will close the valve but such pressure is not reached immediately. The gas inflow rate combined with the size of the control volume set the time taken before the pressure in the control side rises to the threshold level that will close the valve. This arrangement can be used to generate a square pulse of known duration i.e. flow can be on for a fixed time interval, then off. The amplitude of the pulse may also be controlled, for example by a restriction downstream of the valve.
In a proportional control valve, the effective open diameter between the inlet and outlet is decreased in response to rising pressure on the control side and increased in response to rising pressure on the flow side.
As noted above, it is conventional in a valve assembly to clamp the outside perimeter of the disc either transversely between two surfaces or radially within a bore. This permits a seal to be created by the disc i.e. isolates one side of the valve assembly from the other, and holds the periphery stationary leaving the diaphragm free to move during operation. Forces involved in radial compression however will tend to distort the centre of the disc, not just the sealing area providing interference. Moreover, with either sealing method, the fact that the disc is compressed by the clamping causes disc material to flow from the clamped area to the free centre. This causes a distortion or bowing of the disc, which usually extends into the diaphragm area. Most commonly, the diaphragm loses its flatness and adopts a curved, bowed shape. The degree of this bowing, or any other distortion, will vary according to the material and shape of the disc as well as with the level and direction of clamping applied. For example, for the disc described in EP 0606098, the radial compression on the outside of the disc in the assembled valve will cause the centre of the disc to bow, to a level dependent on the tolerances of the bore and the disc.
Regardless of the mechanics by which the disc distorts, the end result is that the distortion or bowing significantly affects operation of the diaphragm in a number of situations. There is uncertainty in the position of the surface of the diaphragm with respect to the seat that it seals. For consistent performance, it is important to ensure that each valve opens and closes at the same threshold pressure. That is, that the response of the central diaphragm area to control pressure is predictable and repeatable. This is particularly significant in valves for which the opening and closing is used to perform a timing function. If the pressure at which a particular valve switches is different, the timing produced for the same volume and bleed flow will vary between valve assemblies. Manufacturing economics require that multiple diaphragms and multiple restrictions can be pre-manufactured to permit mass production of valve assemblies. Differences in performance of either component mean that adjustments will need to be made post-assembly, lengthening considerably the production time. Unfortunately manufacturing tolerances for diaphragms are not below the level for which post-production adjustments are required.
Consistent loading is also important to the predictable function of a proportional type valve.
Clearly the effects of disc distortion or bowing are more significant for smaller discs. Some degree of distortion is inevitable and so to minimise its effect larger discs than required by their application are used, such that the central operable portion is more remote from the area affected by clamping and material flow. However larger discs mean larger valves, which is a distinct disadvantage to portable pneumatic devices.
The significance of the effect of clamping is such that, in practice, normal manufacturing tolerances for moulding the rubber and forming the bore are sufficient to cause significant changes in the behaviour of the sealing area of the disc (diaphragm). For example, a 1% increase in radial or vertical compression can lead to a noticeable difference in bowing. Distortion or bowing may be sufficiently severe that the diaphragm may even be reluctant to shut, or reluctant to open.
Diaphragms for use in pneumatic valves are generally constructed of rubber, although recent developments in thermoplastic elastomer (TPE) technology offer a potential alternative. It is known in the prior art (see, for example, GB 1267920) to include a fabric reinforcing layer within the rubber to strengthen the diaphragm and so to increase its burst strength. This however reduces the diaphragm's flexing ability and hence its sensitivity. The reinforcing layer moreover has limited bearing on the diaphragm's response to compression, and so cannot counteract either the effect of radial sealing or of material flow of the rubber during valve assembly.
There is accordingly a perceived need for a disc component for use in a valve assembly whose operational behaviour is more robust to compression experienced in the valve assembly, such that predictability of valve performance may be improved.